Certificate verification vulnerability in Erlang/OTP 23.2

Posted 2021-01-15 12:06:59.472819

The ssl application version 10.2, part of Erlang/OTP 23.2 and 23.2.1, has a regression in the certificate verification logic. This could result in MitM attacks on clients, as well as unauthorized access to servers that rely on client certificates.

The vulnerability is tracked as CVE-2020-35733. A fix is available as part of ssl-10.2.1, in Erlang/OTP 23.2.2. I will post further details in a follow-up post once everyone has had a chance to upgrade.

The purpose of this post is to help you understand how the issue may affect you. The short version: if you are using an affected version of Erlang/OTP, on development machines, build servers and/or deployment targets, you should probably upgrade as soon as possible.

FAQ

1. Is my application affected?

Certificate verification is primarily used by clients to detect attempts to intercept network traffic or impersonate servers. If your application includes any network clients that use TLS, it is almost certainly affected. Please refer to question 2 for details.

Some servers rely on client certificates to authenticate clients, and they too can be affected. Please refer to question 3 for more information.

2. Are the clients in my application affected?

If the application is running Erlang/OTP 23.2 and it connects to servers using TLS, the answer is almost certainly yes. Some clients do not enable certificate verification, but then such clients would have been susceptible to MitM attacks all along, regardless of Erlang/OTP version.

Some examples of affected clients include:

• HTTP clients, including the built-in httpc, as well as 3rd party clients such as Hackney, HTTPoison, Tesla and Mint; the fact that many of these clients use a custom ‘verify_fun’ (e.g. ssl_verify_fun) does not change that
• SMTP (mail) clients
• MQTT and RabitMQ clients
• Redis clients
• API clients that build on any of the above, e.g. to access AWS, Google APIs, Stripe, Sentry, etc.

Clients that build on the SSH protocol, such as SFTP, are not affected.

3. Is my (web) server at risk?

TLS servers, such as web servers, generally do not perform certificate verification, unless explicitly configured to request a certificate from clients (“mutual TLS”). Therefore in practice most servers are not affected.

Servers that delegate the TLS termination to a reverse proxy (e.g. Nginx) or load balancer (e.g. HAProxy) are not affected, even if they do require client certificates.

However, if your server…

• Relies on client certificates (“mutual TLS”); and
• Uses Erlang/OTP’s built-in ssl application, rather than external TLS termination; and
• Is running Erlang/OTP 23.2 or 23.2.1

…then malicious users may be able to produce fake client certificates that will be accepted by the server. If that is the case, deploy the fix as soon as possible.

4. Is my development/build environment affected?

Possibly. Fetching packages from Hex should be safe, because the Hex client verifies the signature on downloaded packages. But API interactions with the Hex server could be intercepted, which would leak your API credentials. An attacker could then publish malicious modifications to your packages, leading to code execution vulnerabilities for the users of those packages. Do not publish Hex packages until after you have upgraded.

If you are using a tool such as kerl or asdf, uninstall Erlang/OTP 23.2 and 23.2.1, and install 23.2.2 as soon as possible.